In order to understand the underlying mechanisms by which Mg ion, ATP and Ca ion cause muscle to generate tension, the concentrations of these ions in the region surrounding the contractile proteins must be controlled. Unfortunately, the plasma membrane limits the diffusion of these ions, so that the intracellular concentrations cannot normally be controlled. It is possible, however, to chemically remove the membrane with the aid of nonionic detergents, and is a common procedure in striated muscle. Difficulties in applying this technique to smooth muscle have pointed out some of the important differences between the regulatory processes of the two types of muscles. One of these differences is that the calcium sensitivity of the detergent treated smooth muscle is easily lost and can be modified by the type of nonionic detergent used. This suggests that the calcium regulatory system is not bound to a contractile protein but may be found in a superficial part of the muscle. A second difference is that tension responses of detergent treated smooth muscle are modified by micromolar changes in (ATP4-) and corresponding millimolar changes in (Mg ions), when the (MgATP) is kept constant. The sensitivity of the muscle to these changes suggests a role in regulation, not found in striated muscle. BIBLIOGRAPHIC REFERENCES: Gordon, A.R. and Jones, L.R. Tension responses of detergent treated smooth muscle. Biophys. J. 16:210a, 1976. Gordon, A.R. and Jones, L.R. Effect of ATP on Tension in Smooth muscle. Fed. Proc. 35:776, 1976.